Every single Point3D has a Material defining its consistence. Its appearance may change depending on other factors such as the z coordinate.

A material basically contains a string with HTML code which will make up the content of a point's DIV. Additionally it has a JavaScript statement - the refresh() method - that is called when drawing the points, so the material appearance may change.
See also 5.1 3dhtml # Material.
See also 5.5 Materials if you just want to use one of the supplied materials.